/*
 Generates a trail that is always facing upwards using the scriptable mesh interface.
 vertex colors and uv's are generated similar to the builtin Trail Renderer.
 To use it
 1. create an empty game object
 2. attach this script and a MeshRenderer
 3. Then assign a particle material to the mesh renderer
*/
var height = 1.0;
static var genHeight = 1.0;
var time = 2.0;
var alwaysUp = false;
var minDistance = 0.1;

var startColor = Color.white;
var endColor = Color (1, 1, 1, 0);

class TronTrailSection
{
	var point : Vector3;
	var upDir : Vector3;
	var time : float;
}

private var sections = new Array();

function LateUpdate () {
	var position = transform.position;
	var now = Time.time;
	
	if(Time.time > 0.34){
		// Remove old sections
		while (sections.length > 0 && now > sections[sections.length - 1].time + time) {
			sections.Pop();
		}
		
	
		// Add a new trail section
		if (sections.length == 0 || (sections[0].point - position).sqrMagnitude > minDistance * minDistance)
		{
			var section = TronTrailSection ();
			section.point = position;
			if (alwaysUp)
				section.upDir = Vector3.up;
			else
				section.upDir = transform.TransformDirection(Vector3.up);
			section.time = now;
			sections.Unshift(section);
		}
		
		// Rebuild the mesh
		var mesh : Mesh = GetComponent(MeshFilter).mesh;
		mesh.Clear();
		
		// We need at least 2 sections to create the line
		if (sections.length < 2)
			return;
	
		var vertices = new Vector3[sections.length * 2];
		var colors = new Color[sections.length * 2];
		var uv = new Vector2[sections.length * 2];
		
		var previousSection : TronTrailSection = sections[0];
		var currentSection : TronTrailSection = sections[0];
	
		// Use matrix instead of transform.TransformPoint for performance reasons
		var localSpaceTransform = transform.worldToLocalMatrix;
	
		// Generate vertex, uv and colors
		for (var i=0;i<sections.length;i++)
		{
			previousSection = currentSection;
			currentSection = sections[i];
			// Calculate u for texture uv and color interpolation
			var u = 0.0;		
			if (i != 0 )
				u = Mathf.Clamp01 ((Time.time - currentSection.time) / time);
			
			// Calculate upwards direction
			var upDir = currentSection.upDir;
			
			// Generate vertices
			vertices[i * 2 + 0] = localSpaceTransform.MultiplyPoint(currentSection.point);
			vertices[i * 2 + 1] = localSpaceTransform.MultiplyPoint(currentSection.point + upDir * height);
			
			uv[i * 2 + 0] = Vector2(u, 0);
			uv[i * 2 + 1] = Vector2(u, 1);
			
			// fade colors out over time
			var interpolatedColor = Color.Lerp(startColor, endColor, u);
			colors[i * 2 + 0] = interpolatedColor;
			colors[i * 2 + 1] = interpolatedColor;
		}
	
		// Generate triangles indices
		var triangles = new int[(sections.length - 1) * 2 * 3];
		for (i=0;i<triangles.length / 6;i++)
		{
			triangles[i * 6 + 0] = i * 2;
			triangles[i * 6 + 1] = i * 2 + 1;
			triangles[i * 6 + 2] = i * 2 + 2;
	
			triangles[i * 6 + 3] = i * 2 + 2;
			triangles[i * 6 + 4] = i * 2 + 1;
			triangles[i * 6 + 5] = i * 2 + 3;
		}
	
		// Assign to mesh	
		mesh.vertices = vertices;
		mesh.colors = colors;
		mesh.uv = uv;
		mesh.triangles = triangles;
	}
}

@script RequireComponent (MeshFilter)